A tunneling magnetoresistive (TMR) device has a structure in which a tunnel barrier layer is interposed between two ferromagnetic layers. Application of an external magnetic field to the TMR device causes a change in relative angle of magnetization between the two ferromagnetic layers between which the tunnel barrier layer is interposed. This results in change in tunneling conduction probability of electrons through the tunnel barrier layer, and change in resistance of the TMR device. Such TMR devices have been applied to devices such as read-out sensor units of magnetic heads used for hard disks and non-volatile memories utilizing magnetism (MRAMs).
It is known that magnesium oxide (MgO), which is an oxide of magnesium (Mg), is used for a tunnel barrier layer of a TMR device. Known methods for preparing a tunnel barrier layer made of MgO include a method in which the MgO layer is directly formed by high-frequency (RF) sputtering using a MgO target and a method in which an Mg layer is formed and then the MgO layer is formed by an oxidation treatment.
The following methods have been reported for manufacturing a TMR device by using the method in which a Mg layer is formed and then a MgO layer is formed by an oxidation treatment. Specifically, one is a method in which a tunnel barrier layer including a first Mg layer, a MgO layer, and a second Mg layer is formed by, after formation of the first Mg layer, forming the MgO layer on a surface of the Mg layer by natural oxidation, and then forming the second Mg layer (Patent Document 1). Others are a method in which a first Mg layer is formed, followed by an oxidation treatment under high pressure, and then a second Mg layer is formed, followed by an oxidation treatment under low pressure (Patent Document 2), and a method in which a stack of a first MgO layer and a second MgO layer is formed (Patent Documents 3 and 4).
In the method disclosed in Patent Document 3, a tunnel barrier layer including a MgO layer having a crystal orientation is formed by annealing the formed MgO layers in a magnetic field. In the method disclosed in Patent Document 4, a tunnel barrier layer is formed by forming a Mg layer on the first MgO layer, and then raising the temperature to remove unoxidized Mg by vaporization.